This invention relates to an air-fuel ratio control apparatus for an internal combustion engine and, particularly, relates to an air-fuel ratio feedback control apparatus for an internal combustion engine comprising a feedback control-stopping circuit.
There is known a system for precisely controlling the air-fuel ratio in the exhaust gas of an internal combustion engine so that it is a stoichiometric air-fuel ratio. That system comprises an air-fuel ratio detector for example, an oxygen concentration detector disposed in an exhaust system of the internal combustion engine, to detect the oxygen concentration in the exhaust gas. In the system a detection signal from the air-fuel ratio detector is fed back to electronic fuel injection control means to correct the amount of fuel injected in the internal combustion engine so as to effect the above air-fuel ratio control. If a three way catalytic converter capable of simultaneously purifying three harmful components, i.e., HC, CO and NO.sub.x, were to be combined with this air-fuel ratio feedback control system, it would seem to be possible to obtain a clean exhaust gas. However, since, according to this control system the air-fuel ratio is controlled so that it is the stoichiometric air-fuel ratio throughout the entire operating condition of the internal combustion engine, a problem of overheating of the catalytic converter is caused.
The applicant has already proposed an air-fuel ratio feedback control apparatus for an internal combustion engine, which can solve the above problem. This apparatus comprises a mechanism for detecting the level of intake vacuum in an internal combustion engine and the rotational speed thereof, and a feedback-stopping mechanism arranged so that when the detected vacuum level and rotational speed exceed predetermined levels, correction of the air-fuel ratio by the feedback is stopped and the air-fuel correction signal is maintained at a predetermined value. In this apparatus, however, since there is an error in the control range of stopping the feedback, overheating of the catalytic converter cannot be prevented efficiently and assuredly. In addition to this defect, the previously proposed apparatus has a defect in that a circuit must be provided for maintaining the correction signal at a predetermined value at the time of stopping the feedback control. Still further, the apparatus is defective in that it is difficult to reduce the specific fuel consumption.